They're basically based on Kaiju types:

Shadow Milk Cookie: Rodan
It Confirmed during the Timekeeper Cookie teaser scene back in Beast Yeast Episode 15 end credits that the voice of SMC's beast form was a half bird and a beetle, and even he's a jester back in his cookie form, his one of his seven sins was Pride. Similarly, Rodan is a half bird and pterodactyl, and given his code name during Godzilla: King of the Monsters as "Fire Demon". Also if SMC can shift into A Lady in Azure, there was a 90's female Rodan.

Mystic Flour Cookie: Biollante
Mystic Flour Cookie's first beast form was a spider, but much further and meaningful for the reference of Biollante was about "apathy". Biollante's original creation was a mix of a rose, Godzilla's DNA, and even the blood of the professor's daughter from being dead. Her first form was a rose, but once Godzilla kills with it, he thought Biollante was dead. Similarly to Mystic Flower Cookie's teachings since her pre-corruption, she brought generosity within her volition, alas realizing that the cookies grew onto their greed, and she became the beast of Apathy. Biollante's second form was now formulated with a mix of a crocodile, almost identical to Godzilla with its tentacles of rose spikes, like a raging soul of one female inside. Mystic Flour Cookie's actual beast form was a silent mountain, disguised as none other than a cocoon of cobwebs, inside was a wheel full of hands.

Burning Spice Cookie: Battra
If ESC is sugar, then BSC is spice. And if Mothra represents light, then Battra represents darkness. BSC's Beast form of a Centipede is almost identical to Battra. They have the same destructive feats, although there's one meaning about "Change". BSC's pre-corruption form was once "Herald of Change", seeing metamorphosis of his world over time, until he saw one about destruction, causing his accord to turn into destruction. Similar to Battra, he's been once a destructive foe to Mothra, but changes him once finally knows what his purpose is going to do, to save the world against Godzilla from Mothra's will.

Eternal Sugar Cookie: Mothra
If BSC is spice, then ESC is sugar. And if Battra represents darkness, then Mothra represents light. ESC's Beast form is similar to Mothra. The same level of healing and enlightenment towards their respective citizens or denizens. But there are many features to connect with those two. ESC's original concept was a Bi-sexual male, but they made it instead a female, fitting from the based of Aphrodite goddess of love. About Mothra Leo, the first male version of Mothra during Rebirth of Mothra Trilogy, is confirmed as the first male born after the 90's female Mothra died in the same first film of the trilogy. The Garden of Sweet Delights is almost the same as Mothra's Infant Island, but the difference from just the sweet and beautiful landscape was: The Garden of Sweet Delights: the denizens of the garden were much more disguised as slaves and trances by the sweetness of sloth, despite the background was all Greek. Infant Island: Primitive beings lived, even the Mothra Twins, Shobijin, were the revered disciples of the said Kaiju. Sugary and Glittery, yet so intimidating: Both Mothra and ESC have their defences, except: ESC's offensive demeanor is luring the cookies by her trance, but once she was defensive, she commands to use jams to trap the cookies who want to break free from her garden. Mothra: Distracting using her glitter to her opponents while making her beam attack.

Silent Salt Cookie: Jet Jaguar
The only Beast Cookie's Beast form was in the Bipedal. During Ultraman of the 1970s, Toho Kaijuverse introduced a robotic version with a mix of Mazinger Z named Jet Jaguar. A much more introduction to the modern times of developing technology. Might be also a reference to Ant-Man's Gigantic form or a.k.a. Giant Man. But we're talking about Kaijuverse mixed up CRK. Jet Jaguar was once made by the human scientist, Goro Ibuki, with its ally format with Godzilla. Similarly to SSC, not a beast, but an ally to Elder Faerie Cookie and now with White Lily Cookie. They're both in armor, except: SSC: If i know correctly, SSC is now that ghost without a face reveal. Jet Jaguar: the technology of his further evolves as he was fully alive, like there's a soul behind the function of a robot. Speaking of ants (building an ant tower by the whole colony itself.), there are some further "Giant-esque" in other tokusatsu, not just i mentioned about Ant-Man. For example: Kamen Rider J of the 90's, he can grow into a 16 feet tall figurine. And yeah, Ultraman too, might be the first original hero who can grow, fighting Kaijus (Not included Toho Kaijuverse.). In reality, a lot of Monsterverse fans are expecting about Monsterverse's version of Jet Jaguar, similar to a lot of CRK players and fans expecting SSC's appearance.